Servlets are Java technology's answer to Common Gateway Interface (CGI)
programming. They are programs that run on a Web server, acting as a middle
layer between a request coming from a Web browser or other HTTP client and
databases or applications on the HTTP server. Their job is to perform the
following tasks, as illustrated in Figure 21.

Read the explicit data sent by the client. The end user normally
enters this data in an HTML form on a Web page. However, the data could also
come from an applet or a custom HTTP client program.

Read the implicit HTTP request data sent by the browser.Figure 21 shows a single arrow going from the client to the Web server (the layer
where servlets and JSP execute), but there are really two varieties of
data: the explicit data the end user enters in a form 2. and the
behind-the-scenes HTTP information. Both varieties are critical to effective
development. The HTTP information includes cookies, media types and compression
schemes the browser understands, and so forth.

Generate the results. This process may require talking to a
database, executing an RMI or CORBA call, invoking a legacy application, or
computing the response directly. Your real data may be in a relational database.
Fine. But your database probably doesn't speak HTTP or return results in
HTML, so the Web browser can't talk directly to the database. The same
argument applies to most other applications. You need the Web middle layer to
extract the incoming data from the HTTP stream, talk to the application, and
embed the results inside a document.

Send the explicit data (i.e., the document) to the client. This
document can be sent in a variety of formats, including text (HTML), binary (GIF
images), or even a compressed format like gzip that is layered on top of some
other underlying format.

Send the implicit HTTP response data.Figure 21 shows a
single arrow going from the Web middle layer (the servlet or JSP page) to the
client. But, there are really two varieties of data sent: the document
itself and the behind-the-scenes HTTP information. Both varieties are critical
to effective development. Sending HTTP response data involves telling the
browser or other client what type of document is being returned (e.g., HTML),
setting cookies and caching parameters, and other such tasks.

Many client requests can be satisfied by prebuilt documents, and the server
would handle these requests without invoking servlets. In many cases, however, a
static result is not sufficient, and a page needs to be generated for each
request. There are a number of reasons why Web pages need to be built on-the-fly
like this:

The Web page is based on data sent by the client. For instance,
the results page from search engines and order-confirmation pages at online
stores are specific to particular user requests. Just remember that the user
submits two kinds of data: explicit (i.e., HTML form data) and implicit (i.e.,
HTTP request headers). Either kind of input can be used to build the output
page. In particular, it is quite common to build a user-specific page based on a
cookie value.

The Web page is derived from data that changes frequently. For
example, a weather report or news headlines site might build the pages
dynamically, perhaps returning a previously built page if that page is still up
to date.

The Web page uses information from corporate databases or other
server-side sources. For example, an e-commerce site could use a servlet to
build a Web page that lists the current price and availability of each sale
item.

In principle, servlets are not restricted to Web or application servers that
handle HTTP requests but can be used for other types of servers as well. For
example, servlets could be embedded in FTP or mail servers to extend their
functionality. In practice, however, this use of servlets has not caught on, and
I'll only be discussing HTTP servlets.

2.1 The Advantages of Servlets Over "Traditional" CGI

Java servlets are more efficient, easier to use, more powerful, more
portable, safer, and cheaper than traditional CGI and many alternative CGI-like
technologies.

Efficient

With traditional CGI, a new process is started for each HTTP request. If the
CGI program itself is relatively short, the overhead of starting the process can
dominate the execution time. With servlets, the Java virtual machine stays
running and handles each request with a lightweight Java thread, not a
heavyweight operating system process. Similarly, in traditional CGI, if there
are N requests to the same CGI program, the code for the CGI program is
loaded into memory N times. With servlets, however, there would be
N threads, but only a single copy of the servlet class would be loaded.
This approach reduces server memory requirements and saves time by instantiating
fewer objects. Finally, when a CGI program finishes handling a request, the
program terminates. This approach makes it difficult to cache computations, keep
database connections open, and perform other optimizations that rely on
persistent data. Servlets, however, remain in memory even after they complete a
response, so it is straightforward to store arbitrarily complex data between
client requests.

Convenient

Servlets have an extensive infrastructure for automatically parsing and
decoding HTML form data, reading and setting HTTP headers, handling cookies,
tracking sessions, and many other such high-level utilities. Besides, you
already know the Java programming language. Why learn Perl too? You're
already convinced that Java technology makes for more reliable and reusable code
than does Visual Basic, VBScript, or C++. Why go back to those languages for
server-side programming?

Powerful

Servlets support several capabilities that are difficult or impossible to
accomplish with regular CGI. Servlets can talk directly to the Web server,
whereas regular CGI programs cannot, at least not without using a
server-specific API. Communicating with the Web server makes it easier to
translate relative URLs into concrete path names, for instance. Multiple
servlets can also share data, making it easy to implement database connection
pooling and similar resource-sharing optimizations. Servlets can also maintain
information from request to request, simplifying techniques like session
tracking and caching of previous computations.

Portable

Servlets are written in the Java programming language and follow a standard
API. Servlets are supported directly or by a plug-in on virtually every
major Web server. Consequently, servlets written for, say, iPlanet Enterprise
Server can run virtually unchanged on Apache, Microsoft Internet Information
Server (IIS), IBM WebSphere, or StarNine WebStar. They are part of the Java 2
Platform, Enterprise Edition (J2EE; see
http://java.sun.com/j2ee/),
so industry support for servlets is becoming even more pervasive.

Secure

One of the main sources of vulnerabilities in traditional CGI stems from the
fact that the programs are often executed by general-purpose operating system
shells. So, the CGI programmer must be careful to filter out characters such as
backquotes and semicolons that are treated specially by the shell. Implementing
this precaution is harder than one might think, and weaknesses stemming from
this problem are constantly being uncovered in widely used CGI libraries.

A second source of problems is the fact that some CGI programs are processed
by languages that do not automatically check array or string bounds. For
example, in C and C++ it is perfectly legal to allocate a 100-element array and
then write into the 999th "element," which is really some random part
of program memory. So, programmers who forget to perform this check open up
their system to deliberate or accidental buffer overflow attacks.

Servlets suffer from neither of these problems. Even if a servlet executes a
system call (e.g., with Runtime.exec or JNI) to invoke a program on the
local operating system, it does not use a shell to do so. And, of course, array
bounds checking and other memory protection features are a central part of the
Java programming language.

Inexpensive

There are a number of free or very inexpensive Web servers that are good for
development use or deployment of low- or medium-volume Web sites. Thus, with
servlets and JSP you can start with a free or inexpensive server and migrate to
more expensive servers with high-performance capabilities or advanced
administration utilities only after your project meets initial success. This is
in contrast to many of the other CGI alternatives, which require a significant
initial investment for the purchase of a proprietary package.